1. Technical Field
The present invention relates generally to optical amplifying systems. More specifically, the invention relates to optical amplifying systems for high power optical fiber lasers.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Since its introduction in the 1980's, the use of optical fiber in the communications industry has been increasing. Providing a significantly higher bandwidth than its copper wire counterpart, as well as lower losses and less susceptibility to crosstalk, more phone calls are able to be handled and the calls are clearer, especially when over long distances. Today, optical fiber is strung around the globe and serves as a backbone for communications systems such as ground-line telephones, cell phones, cable TV, and networks, including the Internet.
In the 1990's the development of the erbium-doped fiber amplifier (EDFA) further increased the efficiency of the fiber optic communications. The EDFA is an optical amplifier made of a glass fiber doped with the rare earth metal erbium. An optical signal may need to be amplified for a variety of reasons. For example, in long runs of fiber, amplification preserves a signal that has been attenuated through losses occurring along the length of the fiber. Additionally, amplification may be used to enable the signal to operate at higher power levels for high power applications, such as laser printing and etching.
Before the development of the EDFA, amplification of an optical signal involved detecting the optical signal, translating it into an electrical signal, and then amplifying the electrical signal. The amplified electrical signal was then converted back into an optical signal for transmission. While there are still opto-electrical amplifiers in use today, optical amplifiers, such as the EDFA are much more prevalent. Optical amplifiers make use of the physical properties of rare-earth metals such as neodymium, erbium, and ytterbium, for example. These rare-earth metals may be used alone or in combination, such as in an erbium/ytterbium amplifier, and are doped into an optical fiber which serves as both the signal path and the gain medium. Optical energy having wavelengths near 970 nm or 1480 nm from a pump source is absorbed by the rare-earth metal ions and places the ions in a higher energy state. The energized rare-earth metals subsequently transfer energy to a signal traveling through the doped fiber.
Today, optical amplification may be used to enable a signal to operate at much higher power levels than conventional communication systems. Typically, Earth-bound communication systems, like the ground-line telephone or Internet, operate at about 0.2 watts, however, higher power systems may require that the amplifier be able to operate at 10 watts average power and 700 watts peak power, for example. The higher power levels have applications in a variety of fields including: communications between satellites, deep space communications, LIDAR sensing systems, detection systems, laser printing, machining, and etching.
When using an optical amplifier, three optical properties of transition are occurring, namely: 1) spontaneous emission of a photon, 2) stimulated emission of a photon, and 3) absorption. The stimulated emission of photons is the basis for amplification in laser system, but competes with the other two transitions. Ideally, all of the optical power from the pump transfers to the signal through the stimulated emission of photons at the signal wavelength. Throughout the amplification process, however, power may be lost through lasing and spontaneous emission of photons. Lasing and spontaneous emissions are more likely to occur when there is an inversion spike, or stated differently, when a large number of ions suddenly absorb energy and electrons move into higher energy levels. Such an inversion occurs at the front end of erbium/ytterbium amplifiers as pump energy is absorbed by the erbium and ytterbium ions.
In an erbium/ytterbium amplifier, a pump source between approximately 910-990 nm provides optical energy to erbium and ytterbium ions. The ytterbium absorbs the pump energy at a higher rate and a wider wavelength range than the erbium and moves into a higher energy state quickly. The ytterbium in turn transfers energy to the erbium ions. Thus, the ytterbium serves as a catalyst to raise the energy level of the erbium ions quickly. The erbium ions then transfer energy to the signal, thus amplifying the signal. As the signal travels the length of the erbium/ytterbium amplifier, its strength increases as it absorbs more and more of the energy that originated from the pump source.
As described previously, the ytterbium ions absorb pump energy more quickly than the erbium ions. Additionally, at higher power levels energy is transferred from the ytterbium ions to the erbium ions less effectively than at lower power levels causing a buildup of energy in the ytterbium ions and an inversion spike. Therefore, the potential lasing of ytterbium is increased due to the inversion spike at the front end of an amplifier. In particular, the lasing of ytterbium at approximately 1060 nm is a particular risk, and should be precluded. At the elevated power levels, such as above seven watts, parasitic lasing and spontaneous emissions not only reduce the efficiency of the system, but can also lead to hardware damage. Therefore, a system is needed to reduce the front end inversion spike and, thereby, eliminate parasitic lasing and increase the efficiency of the amplifier.